1. Field of the Invention
The present invention relates to an ultrasonic sensor for detecting ultrasonics through a piezoelectric diaphragm, and more particularly, it relates to an ultrasonic sensor which is improved in structure for guiding ultrasonics to a piezoelectric diaphragm.
2. Description of the Background Art
An ultrasonic sensor for detecting breaking of window glass is known as a burglarproof device for an automobile, for example. An example of such an ultrasonic sensor is now described with reference to FIG. 1.
This ultrasonic sensor comprises a case body 1 having an opening in its lower surface, and a base 2 which is mounted on the case body 1 to close the opening. A plurality of through holes are formed in an upper surface of the case body 1, for guiding ultrasonics into the case body 1 therethrough.
On an upper surface of the base 2, on the other hand, support portions 2a are provided integrally with the base 2. A piezoelectric diaphragm 3 is fixed to upper ends of the support portions 2a. This piezoelectric diaphragm 3 is formed as a unimorph type piezoelectric diaphragm by a metal plate 4 and a piezoelectric ceramic plate 5 which is pasted to its lower surface.
An electrode 6 is formed on a lower surface of the piezoelectric ceramic plate 5. The electrode 6 is electrically connected to a terminal 8 by a lead wire 7a, while the metal plate 4 is electrically connected to another terminal 9 by another lead wire 7b.
An equalizer 10 serving as another vibrator is bonded to a central portion of the piezoelectric diaphragm 3 on an ultrasonic receiving side. This equalizer 10 is formed by a rigid material having a conical shape which is open toward the ultrasonic receiving side.
In the aforementioned ultrasonic sensor, a voltage is applied across the terminals 8 and 9 to vibrate the piezoelectric diaphragm, thereby generating ultrasonics. The ultrasonics are reflected by a target, to displace the piezoelectric vibrator 3. A voltage based on this displacement is taken out across the terminals 8 and 9, so that the ultrasonics can be detected.
The aforementioned burglarproof ultrasonic sensor for an automobile must have wide directivity, to be capable of detecting ultrasonics which are caused by every breaking of window glass. The ultrasonic sensor must also have a wide sensitivity frequency band, since the ultrasonics as reflected are varied with the types and broken states of the window glass.
In order to spread the directivity of the aforementioned ultrasonic sensor, the piezoelectric diaphragm 3 may be prepared from that having a small area. If the piezoelectric diaphragm 3 is reduced in area, however, the sensitivity frequency band of the ultrasonic sensor is displaced from that capable of detecting various broken states of window glass, since the resonance frequency of the piezoelectric diaphragm 3 also depends on its dimensions. Further, the bandwidth of a prescribed sensitivity level is narrowed due to separation of two types of combination vibration modes of the piezoelectric diaphragm 3 and the conical equalizer 10. In other words, it is impossible to attain a sufficient sensitivity frequency band.
When the piezoelectric diaphragm 3 is reduced in area, further, sensitivity is reduced due to reduction of the amplitude.